Hydrogen storage is a technology critical to a wide variety of applications, some of the most prevalent being fuel cells, portable power generation, and hydrogen combustion engines. Such applications would benefit substantially from hydrogen storage alloys capable of absorbing and desorbing higher amounts of hydrogen as compared to present day commercially available hydrogen storage alloys. Hydrogen storage alloys having the hydrogen absorption and desorption characteristics of the present invention will benefit such applications by providing longer operating life and/or range on a single charge for hydrogen power generators, fuel cells, and hydrogen internal combustion engines.
In the past considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are being rapidly depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal, natural gas and other hydrocarbons, or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Conventionally, hydrogen has been stored in a pressure-resistant vessel under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas weighs about thirty times the weight of a container of gasoline. Additionally, transfer is very difficult, since the hydrogen is stored in a large-sized vessel. Furthermore, storage as a liquid presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below −253° C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Alternatively, certain metals and alloys have been known to permit reversible storage and release of hydrogen. In this regard, they have been considered as a superior hydrogen-storage material, due to their high hydrogen-storage efficiency. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase metal or alloy system can store large amounts of hydrogen by absorbing hydrogen with a high density and by forming a metal hydride under a specific temperature/pressure or electrochemical conditions, and hydrogen can be released by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time, since they are formed by the insertion of hydrogen atoms to the crystal lattice of a metal. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature/pressure, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas, and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
The hydrogen storage capacity per unit weight of material is an important consideration in many applications, particularly where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of a hydrogen fueled vehicle making the use of such materials. A low desorption temperature is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, fuel cells, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
The prior art hydrogen storage materials include a variety of metallic materials for hydrogen-storage, e.g., Mg, Mg—Ni, Mg—Cu, Ti—Fe, Ti—Mn, Ti—Ni, Mm-Ni and Mm-Co alloy systems (wherein, Mm is Misch metal, which is a rare-earth metal or combination/alloy of rare-earth metals). None of these prior art materials, however, has had all of the properties required for a storage medium with widespread commercial utilization.
Of these materials, the Mg alloy systems can store relatively large amounts of hydrogen per unit weight of the storage material. However, heat energy must be supplied to release the hydrogen stored in the alloy, because of its low hydrogen dissociation equilibrium pressure at room temperature. Moreover, release of hydrogen can be made, only at a high temperature of over 250° C. along with the consumption of large amounts of energy.
The rare-earth (Misch metal) alloys have their own problems. Although they typically can efficiently absorb and release hydrogen at room temperature, based on the fact that it has a hydrogen dissociation equilibrium pressure on the order of several atmospheres at room temperature, their hydrogen-storage capacity per unit weight is only about 1.2 weight percent.
The Ti—Fe alloy system which has been considered as a typical and superior material of the titanium alloy systems, has the advantages that it is relatively inexpensive and the hydrogen dissociation equilibrium pressure of hydrogen is several atmospheres at room temperature. However, since it requires a high temperature of about 350° C. and a high pressure of over 30 atmospheres for initial hydrogenation, the alloy system provides relatively low hydrogen absorption/desorption rate. Also, it has a hysteresis problem which hinders the complete release of hydrogen stored therein.
Hydrogen storage alloys have various crystal structures which play an important role in the alloys ability to absorb and desorb hydrogen. Some of the crystal structures include body centered cubic (BCC), face centered cubic (FCC), or C-14 Laves phase. Hydrogen storage alloys may also change crystal structure upon absorption/desorption of hydrogen. The crystal structure of the BCC phase hydrogen storage alloys, upon absorption of hydrogen, may change to an FCC crystal structure. When this change in crystal structure occurs, excess energy (heat) may be needed to desorb the hydrogen stored within the alloy. Reduced cycling may also be realized due to degradation of the alloy resulting from changes in the crystal structure. Another disadvantage of the change in crystal structure is that the structure does not completely revert back to a BCC crystal structure upon desorption of hydrogen. Upon desorption of hydrogen, the alloy has a combination BCC/FCC crystal structure. This adversely affects the hydrogen storage properties of the alloy, because all the benefits of having a BCC alloy will not be realized. Although the original BCC crystal structure may be restored by heating the alloy, this is not practical for most systems utilizing BCC alloys due to their low temperature design.
BCC alloys are widely used for the storage of hydrogen and have been the subject of multiple patents. Iba et al. (U.S. Pat. No. 5,968,291) discloses Ti—V based BCC phase hydrogen storage alloys comprising two solid solutions having a periodical structure formed by spinodal decomposition. While the alloys disclosed in Iba et al. are able to achieve hydrogen storage capacities of approximately 3.5 weight percent hydrogen, they are only able to achieve approximately 2.0 weight percent reversible hydrogen storage, which makes them unsuitable for many applications. For example, in vehicle applications, alloys having a low reversible hydrogen storage capacity adversely affect the range of the vehicle or require additional weight and space considerations for onboard metal hydride storage to obtain minimum range requirements. Such is the case with portable power applications as well.
Sapru et al. (U.S. Pat. No. 6,616,891) discloses BCC phase hydrogen storage alloys capable of absorbing up to 4.0 weight percent hydrogen while capable of desorbing up to 2.8 weight percent hydrogen. However, Sapru et al. is only able to obtain these hydrogen storage characteristics at temperatures of 150° C. The alloys disclosed by Sapru et al. are Ti—V based with the addition of various modifier elements which improve the reversibility of the hydrogen storage alloys. While the alloys disclosed in Sapru et al. have demonstrated excellent hydrogen absorption/desorption properties at temperatures up to 150° C., there is still a need to provide such properties at lower temperatures. The ability to operate at lower temperatures will provide many additional opportunities for hydrogen to be the fuel of choice for a wide variety of applications.
Another problem with prior art BCC alloys is that while they may initially have a good hydrogen storage capacity, these alloys have very poor stability. Upon increased cycling, the poor stability of the BCC hydrogen storage alloys causes a significant reduction in the hydrogen storage capacity of the alloys, which has resulted in BCC alloys being overlooked for a wide variety of hydrogen storage applications.
Under the circumstances, a variety of approaches have been made to solve the problems of the prior art and to develop an improved material which has a high hydrogen storage efficiency with excellent reversibility, a proper hydrogen dissociation equilibrium pressure, a high absorption/desorption rate, and excellent phase stability resulting in increased cycle life.